There are numerous heat exchangers designed and manufactured using folded fins, and thin, non-round tubes which are then arranged or “stacked” and connected to manifolds (also called headers). These designs have been predominantly used for automotive water-to-air radiators, automotive condensers, truck air charge heat exchangers, automotive heater cores, industrial and truck air-to-oil coolers and more recently, automotive air-conditioning evaporators.
One such condenser is shown in U.S. Pat. No. 4,998,580. A pair of spaced headers has a plurality of tubes extending in hydraulic parallel communication between them and each tube defines a plurality of hydraulically parallel, fluid flow paths between the headers. Each of the fluid flow paths has a hydraulic diameter in the range of about 0.015 to about 0.04 inches. Preferably, each fluid flow path has an elongated crevice extending along its length to accumulate condensate and to assist in minimizing film thickness on heat exchange surfaces through the action of surface tension.
Another such condenser is disclosed in U.S. Pat. No. 6,223,556. The condenser includes two nonhorizontal headers, a plurality of tubes extending between the headers to establish a plurality of hydraulically parallel flow pads between the headers, and at least one partition in each of the headers for causing refrigerant to make at least two passes. An external receiver is also provided to hold refrigerant.
U.S. Pat. No. 5,193,613 discloses a heat exchanger having opposed parallel header tubes having circumferentially spaced grooves formed along the length thereof with inclined sides and a base on the external surface of the groove and spaced annular ribs on the inner surface opposite the grooves. Each groove has a transverse slot therein for receiving open ends of an elongated flat tube. The flat tubes are inserted into the header tubes in a manner which partially blocks the flow path inside the header tubes.
U.S. Pat. No. 5,372,188 discloses a heat exchanger for exchanging heat between an ambient heat exchange medium and a refrigerant that may be in a liquid or vapor phase. The same includes a pair of spaced headers with one of the headers having a refrigerant inlet and the other of the headers having a refrigerant outlet. A heat exchanger tube extends between the headers and is in fluid communication with each of the headers. The tube defines a plurality of hydraulically parallel refrigerant flow paths between the headers and each of the refrigerant flow paths has a hydraulic diameter in the range of about 0.015 to about 0.07 inches. The flow paths may be of varied configurations.
U.S. Pat. No. 4,998,580 discloses a condenser which transfers heat through small hydraulic flow paths. The condenser is for use in automotive applications in which horizontal tubes and small manifolds are used.
Attempts to apply the technology in HVAC&R (Heating, Ventilation, Air-Conditioning and Refrigeration) applications have achieved limited success. Success has been limited because many of the product features, design objectives, and operating issues of HVAC&R applications/equipment are significantly different and more diverse than automotive applications. For example, significant differences may exist in the operating conditions and environments, such as, but not limited to, cooling capacities, operating pressures, air flow rates, energy efficiency, mass flow rates, size of heat exchanger, height to width ratios, oil and refrigerant return, various refrigerants used, operating pressures and temperatures, etc.
Prior conventional heat exchangers, such as those configured for automotive applications which use thin flat tubes (for example, micro-channel tubes) and a brazed manifold structure exhibit deficiencies when provided for use in most HVAC&R applications.
Typical single and multi-pass heat exchanger designs exhibit high refrigerant pressure drops during operation, typically 5 psig or greater. These pressure drops are required to compensate for pressure drop losses in the manifolds or headers. While not an issue in compact automotive designs, where manifold pressure drop can be low, ignored or factored into the single operating design, this pressure drop is not acceptable in HVAC&R applications and can cause other system operating issues. These deficiencies are not apparent until actual field operation experience or test data is taken, and the dynamics and interaction of key operating conditions are better known.
Conventional construction of the manifold header is to use the smallest round material stock size possible (to form the manifolds) to match the tube width, for reasons of lower material cost and for manufacturing reasons associated with integral brazing of the tubes to the manifold. Thus, for a tube that is 1 inch wide, a 1 inch inside diameter manifold or header is typically used. While this particular size combination may generally be usable for automotive applications, allowing for good automated insertion of the tube into the header and stopping point for the tube, it is generally not suitable, and many times not appropriate, for most HVAC&R applications. That is, for broad-based use in HVAC&R applications, this or similarly sized manifold diameters, and more specifically, “useable cross sectional internal area” imposes significant operational limitations regarding the capacity and capacity range of the heat exchanger, and also induces major performance issues and losses due to pressure drop in the manifold or header, as well as refrigerant and oil entrapment in the manifold area. In condensers, this tube/manifold size combination corresponds to about a 5 percent to about a 20 percent operating capacity loss at various refrigerant flow conditions. In evaporators, this tube/manifold size combination results in a loss of operating capacity that can easily exceed 30 percent.
The pressure drop of refrigerant and fluids in the conventional manifolds or headers is one of several phenomena that can induce mal-distribution of refrigerant vapor entering the tubes. Mal-distribution may occur in heat exchangers functioning as condensers or evaporators. In condensers, an increase in the manifold pressure (or pressure drop) results in less refrigerant being provided to tubes positioned further from the inlet of the manifold or header. The effect can be worsened for multi-pass arrangements, depending upon the number of tubes, mass flow rate of refrigerant, or for other reasons. Imposing additional increase in pressure (or pressure drop) through the use of multi-passes can help compensate or partially correct the mal-distribution in condensers, but results in a significant additional refrigerant pressure drop and loss of heat transfer capacity of the heat exchanger. In evaporators, multi-pass arrangements can induce mal-distribution that increasingly occurs in each fluid flow pass through the tubes. In single pass evaporators, mal-distribution of refrigerant can be induced both in the entrance manifold or header and exiting manifold or header.
One way to avoid mal-distribution in condensers (and evaporators) has been to provide extremely low manifold header pressure losses as a ratio of tube pressure drop losses. In evaporators, the ratio of exit pressure drop due to the exiting manifold versus the pressure drop due to the tubes can be an important consideration. That is, the tubes near the connection may be subjected to a reduced pressure drop when compared to the pressure drop of the tubes positioned further away from the connection. For example, if the manifold has a one psi pressure drop over its length, and the tubes have a two psi pressure drop, the tubes closest to the exit connection will have more refrigerant flow than the tubes positioned further from the connection. Since the mass fluid flow rate is exponentially related to the induced pressure drop, the pressure drop over the length of the manifold may cause an imbalance of the amount of fluid being evaporated in each tube.
Conventional micro-channel tube heat exchangers have unpredictable performance due to internal manifold baffling. Tube pressure drop losses combined with manifold pressure drop losses in multi-pass designs require extremely complex calculations and analysis in order to predict both full load and part load performance of the heat exchanger. In addition, variations in the overall refrigerant charge in the refrigeration system, or “back up” of refrigerant in the condenser at full and/or partial load, can render all analysis and prediction, tenuous, if not unreliable. Thus, the refrigerant charge level can significantly affect the available condenser heat transfer (internal tube) surface and thus, refrigeration system capacity and energy use. In other words, the provision of a predetermined amount of refrigerant (versus “over-charging” or “under-charging” or loss of refrigerant over time) can adversely affect efficient operation of the heat exchanger, and the refrigerant system.
Because of the relatively small ratio of manifold or header cross sectional area to tube cross sectional area and manifold header to overall system capacity in the current state of the art heat exchangers, there is typically insufficient refrigerant holding charge in a conventional condenser having “micro-channel” tubes. Without the use of an additional component called a refrigerant receiver, the refrigeration system is thus said to be “critically charged”. That is, a very small addition of refrigerant to the system may cause the condenser to “back up” with refrigerant inside the “micro-channel” tubes, thus reducing the amount of heat transfer surface, thereby increasing the condensing pressure (causing loss of system capacity and/or higher energy consumption). On the other hand, a loss of refrigerant or under-charge in a critically charged system can cause the evaporator to have insufficient refrigerant, resulting in reduced evaporator temperatures, which in turn results in loss of refrigeration capacity, and/or higher energy use, and/or potential freezing of water condensate on the air coil, (or water being cooled inside a refrigerant-to-water type evaporator). In some cases, the low evaporator temperatures result in system safety shut-down or possible evaporator rupture/failure. Thus, in the state of the art heat exchanger constructions or designs having “micro-channel” tubes, also referred to as “micro-channel” heat exchangers, users have discovered, when applied to typical HVAC&R equipment and system designs, there exists a narrow range of refrigerant volume (refrigerant charge) for a particular refrigerant system, in which if the refrigerant volume is outside of the range of refrigerant volume, that is, too much or too little refrigerant charge, can result in unexpected or adverse operations of the system, or possibly system failure.